Despite inducing radiographic regressions in patients whose lung adenocarcinomas harbor somatic mutations in the exons encoding the kinase domain of EGFR, the EGFR tyrosine kinase inhibitors (TKIs), gefitinib and erlotinib, rarely induce complete responses and do not cure patients. This suggests that not all lung adenocarcinoma cells "respond" by undergoing cell death and/or that there are potential defects in pathways within cells that prevent complete tumor elimination. Consistent with this, in our preliminary studies, we have identified four human EGFR-mutant cell lines that appear "addicted" to mutant EGFR, i.e., they display nanomolarsensitivity to erlotinib; however, while three of these cell lines undergo apoptosis when exposed to erlotinib, one line does not. The goal of this project - which is a logical extension ofour published work - is to understand further why EGFR mutant cells persist after initial treatment with EGFR TKIs. These studies will be bolstered by cooperation with the other co-investigators in this application, through the sharing of data, protocols, and reagents, and the formation and use of core facilities. Using human lung adenocarcinoma cell lines and xenografts, human lung adenocarcinomas, and mouse lung adenocarcinomas, all of which harbor EGFR kinase domain mutations, and a variety of molecular, biochemical, and pharmacological approaches, our specific aims are to: 1) define the mode of response to EGFR TKIs in drug-"sensitive" EGFR mutant cells, 2) identify collateral targets in erlotinib-sensitive EGFR mutant cells that could be manipulated to maximize tumor cell killing, and 3) correlate our findings from Aims 1 and 2 with data from a unique collection of human lung adenocarcinoma specimens obtained from patients with "persistent" disease, i.e., with EGFR-mutant cancer cells remaining after initial short-term treatment with EGFR TKIs. An enhanced understanding of the molecular events involved in "persistent" disease could 1) lead to the identification of new targets to optimize tumor cell death, 2) provide insight into subsequent mechanisms of acquired resistance to EGFR TKIs, and 3) establish a framework upon which to prioritize the study of new anti-apoptotic drugs emerging in the clinic.